Peter, no question. Stewie is funny, but Peter is the star of the show, and because of that, he gets all the best stuff.
But when Stewie & Peter are in the same scene, its GOLD, JERRY, GOLD! Especially the scene where he's trying to get him to Potty Train?
Stewie: Oh yeah, in that case, why don't we just light up a doobie and watch porn?
Peter: Uhmm........yeah?